<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        var a = 123.9; 
        var b = "hello     world";
        document.write(b);
        console.log(b);

        var c = true;
        var d = false;
        document.write(d);

        var num = undefined;
        document.write(num);
        var tr = null;
        document.write(tr);


        var a = 10;
        a = "nihao";  // 可以但不推荐
    </script>
</body>
</html>